The present invention relates to the field of liquid crystal display devices employed in information processing terminals and video appliances, and their manufacturing methods. More particularly, the present invention relates to active matrix liquid crystal display devices with a high aperture ratio using an active element substrate.
To increase the aperture ratio of an active matrix liquid crystal display devices, pixel electrodes are formed on the uppermost layer of each thin film layer forming active elements. This type of liquid crystal display device and its manufacturing method are disclosed for example in Super-High-Aperture-Ratio TFT-LCD Structure, Digest of Technical Papers 1996 International Workshop on Active Matrix Liquid Crystal Displays (AM-LCD ""96), pp. 149-152 by J. H. Kim et. al.
FIG. 5A shows a perspective plan view of a single pixel of the disclosed liquid crystal display device. FIG. 5B is a sectional view taken along the dotted line 5Bxe2x80x945B in FIG. 5A. The liquid crystal display device shown in FIGS. 5A and 5B, includes a first substrate (a) which holds the active elements, a gate electrode (b), a gate insulation layer (c), a channel layer (d), source electrode wiring (e), and a drain electrode (f) forming a thin film transistor TFT. An insulation layer (g) is shown on the TFT. A contact hole (g1) for connecting the drain electrode (f) and pixel electrode (h) is formed in the insulation layer (g). A second substrate (i) for sandwiching a liquid crystal layer (k) between the the substrates has a light-shielding layer (j) (black matrix). The light-shielding layer (j) blocks the light from a portion of the liquid crystal layer (k) where electrical control of the pixel electrode (h) is incomplete.
For manufacturing such liquid crystal display device, first, the gate electrode (b) is formed on a glass first substrate (a). Then, the gate insulation layer (c), containing SiN and a-Si, and channel layer (d) are formed. After forming the source electrode wiring (e) and drain electrode (f), an insulation layer (g) having a contact hole (g1) is formed on the drain electrode (f) using low dielectric organic material (dielectric constant: 2.6-2.7) such as benzocyclobutane. A pixel electrode (h), connected to the drain electrode (f) through the contact hole (gl) is formed over the insulation layer (g) partially overlapping the source electrode wiring (e). The second substrate (i) with the light-shielding layer (j) is placed facing the first substrate (a), and the liquid crystal layer (k) is injected in-between to complete the liquid crystal display device.
The insulation layer (g) as provided above enables to extend the pixel electrode (h) up to the dotted line in FIGS. 5A and 5B, and up to the source electrode wiring (e) shown in FIG. 5A, while maintaining insulation. This allows to expand the liquid crystal driving area of the pixel electrode (h), resulting in an increased aperture ratio. Furthermore, by forming the insulation layer g with a low dielectric organic material, the parasitic capacitance between the pixel electrode (h) and source electrode wiring (e) may be reduced. This enables the achievement of a liquid crystal display device with high aperture ratio and low occurrence of cross talk.
However, the configuration of the conventional liquid crystal display device as described above may generate a crack in the pixel electrode (h), starting from the contact hole (g1), and cause a pixel defect in the liquid crystal display device. This phenomenon was investigated by means of a series of detailed experiments, and the following details were discovered.
As shown in FIG. 6, cracks (l) on the pixel electrode (h) tend to start from corners of the via hole along the contact hole (g1). They almost never start from a straight section. The crack is also likely to extend in the direction of the shortest distance between the contact hole (g1) and the edge of the pixel electrode (h). This behavior suggests that the crack occurs due to the formation of the pixel electrode (h) on the insulation layer (g) made of organic material. Since a heating process is used for the formation of the pixel electrode (h), the cracks (l) are thought to result from the difference in stress between the insulation layer (g) and pixel electrode (h).
The contact hole (g1) on the drain electrode (f) is normally nearly round, as shown in the conventional example in FIG. 5A. A regular square pattern is generally used as a photomask pattern for forming contact hole (g1). The pattern at the corners of the contact hole is not sharp and the corners of the contact hole become rounded as shown in FIG. 5A because the insulation layer g is relatively thick compared to the size of the contact hole. This is because it is necessary to minimize the area needed to create the contact hole (g1) on the drain electrode (f) for connecting the drain electrode (f) and the pixel electrode (h) and thus maintain the largest possible aperture ratio. The shape of the contact hole (g1) for electrically connecting the pixel electrode (h) and drain electrode (f) is usually minimum size, with the contact hole (g1) having a small radius of curvature for its open rim.
Accordingly, a crack from a via hole in the pixel electrode (h) along the contact hole (g1) may propagate in any direction. If several cracks (la) reach the edge of the pixel electrode (h) from almost the same starting point, for example, as shown in FIG. 6, these cracks may separate a portion of the pixel electrode (h) from the connecting part with the drain electrode (f), generating a defective electrode area (h1) which has defective electrical connection with the drain electrode (f). In the defective electrode area (h1), a portion which is not covered by a light-shielded area including the light-shielding layer (j), drain electrode (f), and source electrode wiring (e) becomes a defective pixel area (h2), shown by the slanted lines in the defective electrode area (h1). The defective pixel area (h2) becomes visibly obvious when the liquid crystal display device is being driven.
The present invention aims to provide a liquid crystal display device with a high aperture ratio which eliminates defective pixel areas even if a crack occurs in a pixel electrode. The present invention further provides a method for manufacturing such liquid crystal display device.
A liquid crystal display device of the present invention includes an active element; a contact electrode connected to the active element; a pixel electrode; an insulation layer insulating the active element, and the contact electrode from the pixel electrode; and a contact hole provided in the insulation layer for connecting the contact electrode and pixel electrode. The contact hole has more than one radius of curvature. All portions of the pixel electrode close to small radiuses of curvature out of the aforementioned multiple radiuses of curvature of the contact hole are all located in a light-shielded area.
Moreover, the liquid crystal display device of the present invention includes the active element; a contact electrode connected to the active element; a pixel electrode; an inter-layer insulation layer for insulating the active element, and the contact electrode from the pixel electrode; a contact hole provided in the interlayer insulation layer for connecting the contact electrode to the pixel electrode. The contact hole has a major axis longer than another, minor axis perpendicular to the major axis. The portion of the pixel electrode close to the extended line of the major axis of the contact hole is located in the light-shielded area.
Still more, the liquid crystal display device of the present invention has an active element containing substrate that includes an upper layer over which the pixel electrode is located and the pixel electrode is connected to the active element under an interlayer insulation layer through a contact hole provided on the interlayer insulation layer beneath the upper layer. The contact hole has an oval, ellipse, peanut, or other asymmetric shapes having a major axis. A portion of the pixel electrode extending from the contact hole area along the major axis of the contact hole to adjacent edges of the pixel electrode is located in the light-shielded area.
The contact hole in the interlayer insulation layer has a shape wherein some portions have smaller radiuses of curvature than other portions. Or, the contact hole has a major axis which is longer than another, minor axis perpendicular to the major axis. Or, the contact hole has an oval, ellipse, peanut, or asymmetric shape having a major axis. In such contact holes, the location of small radiuses of curvature of its open rim will be limited. Accordingly, the occurrence of a crack may be limited to the location of small radiuses of curvature. addition. The the direction of the extension of the crack may be regulated because it has been found that it preferentially leads toward nearby edges of the pixel electrode. This observation permits to identify on the pixel electrode an area exhibiting a high probability for a crack to develop. Such crack will likely extend from the smallest radius in the hole rim, to the closest electrode edge.
A portion of the pixel electrode which extends from the contact hole to an adjacent edge of the pixel electrode roughly along the major axis direction of the contact hole matches (this limited) preferred direction of the crack. This enables to avoid a defective electrode area, even if it occurs, from being a visible pixel defective area which can be externally recognizable when driving the liquid crystal, because the defective electrode area is located inside the light-shielded area.
Most of the portion of the pixel electrode extending from the contact hole to the adjacent edges roughly along the major axis of the contact hole is already located in the light shielded area in the conventional designs. Therefore, the present invention does not reduce the aperture ratio of the active element substrate even if this area is specified as the light-shielded area.
Furthermore, the contact hole may be disposed at the corner close to the active element of the pixel electrode in a direction that the major axis of the contact hole crosses both edges making the corner. If the major axis of the contact hole is positioned perpendicular to a straight line bisecting an angle created by both edges of the pixel electrode at the corner of the pixel electrode, the major axis portion may be made smaller, enabling to further preferably prevent the reduction of the aperture ratio.
If the light-shielded area is an area shaded by the light-shielding layer and at least one of electrodes of the active elements, the reduction of the aperture ratio for securing alignment allowance of the light-shielding layer is preventable, realizing high aperture ratio.
If the active element is made of a thin film transistor, and a specified electrode for connecting the active element and pixel electrode is the drain electrode, cross talk may be further reduced.
If the interlayer insulation layer is made of an organic material, a thick interlayer insulation layer with low dielectivity may be formed, enabling to reduce a cross talk between electrodes.
If the pixel electrode is made of indium tin oxide, a low-resistance and high transmissivity electrode may be formed without damaging the active elements and interlayer insulation layer.
Still in accordance with this invention there is contemplated a method for manufacturing the liquid crystal display device of the present invention. The method includes the following steps forming an active element including an electrode connected to the active element on a substrate; forming an interlayer insulation layer having a contact hole which has more than one radius of curvature over the electrode of the active element; and forming a pixel electrode connected to the electrode through the contact hole in a way that portions of the pixel electrode close to smaller radiuses of curvature out of multiple radiuses of curvature of the contact hole are all located in a light-shielded area.
Still within the scope of the present invention, the method for manufacturing the liquid crystal display device of the present invention includes the following steps of: forming the active element and electrode connected to the active element on the substrate; forming the interlayer insulation layer having the contact hole which has a major axis longer than another axis perpendicular to the major axis and the major axis is located over a predetermined electrode of the active element; and forming the pixel electrode connected to the predetermined electrode through the contact hole in a way that the pixel electrode portion close to an extended line along the major axis of the contact hole is located in the light-shielded area.
Still more, the method for manufacturing the liquid crystal display device of the present invention includes the steps of: forming the electrodes such as those required for the active element and wiring on the substrate; forming the interlayer insulation layer having an approximately oval contact hole over a desired electrode of the active element; and forming the pixel electrode connected to the desired electrode through the contact hole on the interlayer insulation layer in a way that its major axis portion of the pixel electrode extending from the contact hole to an adjacent edge- roughly along the major axis is located in the light-shielded area.
Furthermore, the method for manufacturing the liquid crystal display device of the present invention features another step of forming a light-shielding layer in a way to form a light-shielded area to cover the portion of the pixel electrode which extends from the contact hole to the adjacent edges roughly along the major axis direction.
These manufacturing methods enable to fabricate the liquid crystal display device with the above conventional characteristics by just changing the shape of the contact hole and the positional relationship of the pixel electrode and contact hole.
Furthermore, the method for manufacturing the liquid crystal display device of the present invention includes the steps of forming the pixel electrode connected to the predetermined electrode of the active element through the contact hole on the interlayer insulation layer in a way to block the light from at least a part of the major axis portion extending from the contact hole by overlaying the predetermined electrode; and forming the light-shielding layer to create the light-shielded area to cover another part of the major axis portion which is not covered by the predetermined electrode. With this method, the liquid crystal display device in which the light-shielded area is blocked the light by the light-shielding layer and at least one of electrodes of the active element. Also in these manufacturing methods, it is preferable to employ a thin film transistor for the active element, the drain electrode for the desired predetermined electrode, an organic material for the interlayer insulation layer, and indium tin oxide for the pixel electrode.
The above characteristics of the present invention are detailed in the following description and drawings. Each characteristics of the present invention may be applied independently or in combinations.